AngularJS lets you write client-side web applications as if you had a smarter browser. It lets use good old HTML (or HAML, Jade and friends!) as your template language and lets you extend HTML’s syntax to express your application’s components clearly and succinctly
Give your JS App some Backbone with Models, Views, Collections, and Events
Select2 is a jQuery based replacement for select boxes. It supports searching, remote data sets, and infinite scrolling of results.
The official jQuery user interface library.
Ajax.org Cloud9 Editor
The HTML Presentation Framework
Chosen is a library for making long, unwieldy select boxes more friendly.
In-browser code editor
A package manager for the web, created at Twitter